Bug 1749522 - When plain text encoding speculation fails, restart the plaintext mode of the tokenizer. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D135830

parser/html/nsHtml5StreamParser.cpp

@@ -1869,6 +1869,13 @@ void nsHtml5StreamParser::DiscardMetaSpeculation() {
   mTokenizer->setLineNumber(1);
   mLastWasCR = false;
 
+  if (mMode == PLAIN_TEXT || mMode == VIEW_SOURCE_PLAIN) {
+    // resetToDataState() above logically rewinds to the state before
+    // the plain text start, so we need to start plain text again to
+    // put the tokenizer into the plain text state.
+    mTokenizer->StartPlainText();
+  }
+
   mFirstBuffer = mLastBuffer;
   mFirstBuffer->setStart(0);
   mFirstBuffer->setEnd(0);